Actually, it's always safe to play around with as little site debt as possible because there are so many things that can go wrong when running a site. Also, it's not easy to sell advertising because most ads purchased are cheap ads to promote ptp and there are sites selling ads for real cheap. So basically, site debt is site debt. By the time members reach payout, you will have the sign up bonus debt + the amount the members earned.
I really do not think it is advisable (sp?) to offer high sign up bonuses. I would understand offering 5 cents or 10 cents maximum, but definitely not $3.
Anyway, good luck with your program